Interferometric encoder systems

ABSTRACT

A method for determining information about changes along a degree of freedom of an encoder scale includes directing a first beam and a second beam along different paths and combining the first and second beams to form an output beam, where the first and second beams are derived from a common source, the first and second beams have different frequencies, where the first beam contacts the encoder scale at a non-Littrow angle and the first beam diffracts from the encoder scale at least once; detecting an interference signal based on the output beam, the interference signal including a heterodyne phase related to an optical path difference between the first beam and the second beam; and determining information about a degree of freedom of the encoder scale based on the heterodyne phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application No.61/319,252, filed on Mar. 30, 2010, Provisional Application No.61/327,983, filed on Apr. 26, 2010, and Provisional Application No.61/422,482, filed on Dec. 13, 2010. The contents of each of theseprovisional applications are hereby incorporated by reference in theirentirety.

BACKGROUND

The disclosure relates to interferometric encoder systems and methods,and applications for the encoder systems and methods.

In some cases, interferometric measuring systems monitor changes in therelative position of a measurement object based on an opticalinterference signal. For example, an interferometer generates theoptical interference signal by overlapping and interfering a measurementbeam reflected from the measurement object with a second beam, sometimescalled a “reference beam,” where the measurement beam and the referencebeam are derived from a common source. Changes in the relative positionof the measurement object correspond to changes in the phase of themeasured optical interference signal.

An example of such interferometric measuring systems are interferometricencoder systems, which evaluate the motion of an object by tracking ameasuring graduation, called the encoder scale. Typically, aninterferometric encoder system includes the encoder scale and an encoderhead. The encoder head is an assembly that includes an interferometer.The interferometer directs a measurement beam to the encoder scale,where it diffracts. The interferometer combines the diffractedmeasurement beam with a reference beam to form an output beam thatincludes a phase related to the position of the object. Encoder systemsare used extensively in lithographic applications for monitoring themotion of moveable stages in a lithography tool. Encoder systems can beadvantageous in such applications due to their relative insensitivity toatmospheric turbulence.

SUMMARY

The disclosure relates to encoder systems and methods to implement aheterodyne measurement of phase changes occurring in reflected ortransmitted beams diffracted from encoder scales resulting from themotion of the encoder scale in specific measurement directions. Theencoder systems may be arranged to generate an interferometric signalbased on a diffracted measurement beam in a non-Littrow configuration.The encoder systems can include compact encoder heads and offer multiplemeasurement channels.

In certain aspects, the disclosure features an encoder system capable ofmeasuring accurately changes in one or more displacement directions ofan encoder scale including: (1) a source beam of a frequency stabilizedillumination with two linear orthogonally polarized components withdifferent frequencies; (2) means (e.g., an optical assembly), fordirecting one or both components onto a encoder scale attached to thebody to be monitored; (3) means (e.g., an optical assembly), forreceiving one or both components of the diffracted beams; (4) means(e.g., an optical assembly), for combining and mixing both frequencycomponents to produce a heterodyne signal; (5) means (e.g., a detectormodule including a photoelectric detector) for producing an electricalmeasurement signal; and (6) means (e.g., a phase meter/accumulator) forindicating the measured phase, the measured phase being related to theencoder scale's diffractive structure and the displacement of theencoder scale along the sensitive directions. Embodiments can includeone or more of the following features. For example, embodiments may bedesigned so they do not operate at Littrow, to operate with a heterodynelaser source and detection means, may be first-order insensitive to tipand tilt of the encoder scale, may provide a minimum of two axis ofmetrology (e.g., X and Z, Y and Z), and/or can function with 2D encoderscales to provide full 3D motion detection if desired. The encodersystems can be used in lithography tools, but could be used for otherapplications too.

Various aspects of the invention are summarized as follows.

In general, in a first aspect, the invention features methods fordetermining information about changes along a degree of freedom of anencoder scale, the methods including: directing a first beam and asecond beam along different paths and combining the first and secondbeams to form an output beam, where the first and second beams arederived from a common source, the first and second beams have differentfrequencies, where the first beam contacts the encoder scale at anon-Littrow angle and the first beam diffracts from the encoder scale atleast once; detecting an interference signal based on the output beam,the interference signal including a heterodyne phase related to anoptical path difference between the first beam and the second beam; anddetermining information about a degree of freedom of the encoder scalebased on the heterodyne phase.

Implementations of the methods can include one or more of the followingfeatures. For example, the first beam can be normally incident on theencoder scale. The first beam can be non-normally incident on theencoder scale. The first beam can be incident on the encoder scale at anangle so that the diffracted measurement beam is normal to the encoderscale.

The degree of freedom can be a position of the encoder scale along anaxis that lies in the plane of the encoder scale.

The first and second beams can be linearly polarized beams. The path ofthe first beam before and after diffracting from the encoder scale candefine a plane and the first beam can be polarized orthogonal to theplane. In some embodiments, the encoder scale can include grating linesthat extend along a first direction and the measurement beam is linearlypolarized in a direction parallel to grating lines. Directing the firstand second beams can include rotating a polarization state of the firstbeam by 90° prior to diffraction from the encoder scale.

In certain embodiments, the first and second beams are orthogonallypolarized beams. Detecting the interference signal can include directingthe output beam through a polarizing element that transmits a componentof each of the orthogonally polarized first and second beams.

Directing the first and second beams along different paths can includederiving the first and second beams from an input beam using a beamsplitter. The first and second beams can be combined using the beamsplitter. In some embodiments, the first and second beams are combinedusing a second beam splitter. The beam splitter can be a polarizing beamsplitter.

In some embodiments, the second beam does not contact the encoder scale.Alternatively, the second beam can diffract from the encoder scale atleast once. The diffracted second beam can be a zero-order diffractedbeam. The diffracted second beam can be co-linear with the first beam atthe encoder scale.

The first beam can diffract from the encoder scale only once.Alternatively, in some embodiments, the first beam diffracts from theencoder scale more than once (e.g., twice). A path of the first beamprior to diffracting from the encoder scale can be parallel to a path ofthe first beam after diffracting from the encoder scale a second time.After diffracting from the encoder scale a first time, the first beamcan be directed by a retroreflector to diffract from the encoder scale asecond time prior to being combined with the second beam.

The information can be derived based on more than one heterodyne phasemeasurements.

The method can further include combining a third beam and a fourth beamto form a second output beam, the third and fourth beams being derivedfrom the common source and the third beam diffracts from the encoderscale at least once; and detecting a second interference signal based onthe second output beam, the second interference signal comprising aheterodyne phase related to an optical path difference between the thirdbeam and the fourth beam. The first and third beams can contact theencoder scale at the same location. Alternatively, or additionally, thefirst and second beams can contact the encoder scale at differentlocations. The information can be determined based on the heterodynephases of the first and second output beams. The once-diffractedmeasurement beam and the third beam can be the +1 and −1 diffractedorders of the measurement beam, respectively.

The degree of freedom can correspond to a displacement of the encoderscale along a first axis in a plane of the encoder scale. The method caninclude determining information about a second degree of freedom of theencoder scale. The second degree of freedom can be a displacement of theencoder scale along a second axis orthogonal to the first axis. Thesecond axis can be in the plane of the encoder scale. The second axiscan be orthogonal to the plane of the encoder scale.

The degree of freedom can be a tilt of the encoder scale about an axis.

The method can include monitoring a reference phase of an input beamproduced by the common source and from which the first beam is derived.Determining the information can include comparing the heterodyne phaseto the reference phase.

The first beam can have a wavelength in a range from 400 nm to 1,500 nm.In some embodiments, the first beam has a wavelength of about 633 nm orabout 980 nm.

The encoder scale can include a grating. The grating can have a pitch ina range from about 1λ to about 20λ, where λ is a wavelength of the firstbeam. In some embodiments, the grating has a pitch in a range from about1 μm to about 10 μm.

The first and second beams can be directed along their respective pathsusing an optical assembly and the method further comprises translatingthe encoder scale relative to the optical assembly while determining theinformation. The optical assembly or encoder scale can be attached to awafer stage and the method further includes monitoring the position of awafer relative to radiation from a lithography system based on theinformation. The optical assembly or encoder scale can be attached to areticle stage and the method further includes monitoring the position ofa reticle relative to radiation from a lithography system based on theinformation.

In general, in another aspect, the invention features encoder systemsthat include: an optical assembly configured to derive a first beam anda second beam from an input beam, direct the first and second beamsalong different paths and combining the first and second beams to forman output beam, where the first and second beams have differentfrequencies; a diffractive encoder scale positioned in the path of thefirst beam so that the first beam contacts the diffractive encoder scaleat a non-Littrow angle and the first beam diffracts from the diffractiveencoder scale at least once; a detector positioned to detect the outputbeam; and an electronic processor configured to receive an interferencesignal from the detector, the interference signal including a heterodynephase related to an optical path difference between the first and secondbeams, and determine the information about a degree of freedom of theencoder scale based on the heterodyne phase.

Embodiments of the encoder system can include one or more of thefollowing features and/or features of other aspects. For example, theoptical assembly can include an optical element that splits the inputbeam into the first and second beams. The optical element can be anon-polarizing beam splitter or a polarizing beam splitter. The opticalelement can combine the first and second beams to form the output beam.

The optical assembly can include one or more optical elements configuredto split the input beam into two parallel sub-input beams prior toderiving the first and second beams. The optical assembly can include abeam splitter configured to split one of the sub-input beams into thefirst and second beams and the split the other sub-input beam into athird and fourth beam, wherein the optical assembly directs the thirdand fourth beams along different paths and combines the third and fourthbeams to form a second output beam. The optical assembly can direct thethird beam to diffract from the encoder scale at least once. The opticalassembly can include two retroreflectors positioned to reflect theonce-diffracted first and third beams, respectively, to diffract fromthe encoder scale a second time. The first and third beams contact theencoder scale at different locations. The encoder scale can diffract thefirst and third beams into the +1 and −1 diffraction orders,respectively. The path of the input beam at the optical assembly can beparallel to paths of the first and second output beams. The path of theinput and first and second output beams can be parallel to a plane ofthe encoder scale.

The optical assembly can include a half wave plate in the path of thefirst beam.

The encoder scale can include a grating (e.g., a one-dimensional or atwo-dimensional encoder scale).

In a further aspect, the invention features systems that include amoveable stage; and the encoder system of the foregoing aspect, whereineither the encoder scale or the optical assembly are attached to thestage.

In general, in another aspect, the invention features encoder systemsthat include a means for deriving a first beam and a second beam from aninput beam where the first and second beams have different frequencies;a means for directing the first and second beams along different paths;a means for combining the first and second beams to form an output beam;a diffractive encoder scale positioned in the path of the first beam sothat the first beam contacts the diffractive encoder scale at anon-Littrow angle and the first beam diffracts from the diffractiveencoder scale at least once; a means for detecting the output beam; anda means for receiving an interference signal from the detector, theinterference signal comprising a heterodyne phase related to an opticalpath difference between the first and second beams, and determine theinformation about a degree of freedom of the encoder scale based on theheterodyne phase.

Embodiments of the encoder system can include one or more features ofother aspects.

In general, in a further aspect, the invention features encoder systemsthat include an optical assembly configured to derive a first beam and asecond beam from an input beam, wherein the first and second beams arelinearly polarized beams having different frequencies, the opticalassembly being further configured to direct the first and second beamsalong different paths and combine the first and second beams to form anoutput beam, the optical assembly including an optical elementpositioned in the path of the first beam and configured to rotate thelinear polarization state of the first beam by 90°; a diffractiveencoder scale positioned in the path of the first beam so that the firstbeam diffracts from the diffractive encoder scale at least once; adetector positioned to detect the output beam; and an electronicprocessor configured to receive an interference signal from thedetector, the interference signal including a heterodyne phase relatedto an optical path difference between the first and second beams, anddetermine the information about a degree of freedom of the encoder scalebased on the heterodyne phase.

Embodiments of the encoder system can include one or more of thefollowing features and/or features of other aspects. For example, theoptical element can intersect the path of the first beam twice androtates the linear polarization state of the first beam by 90° eachtime. The optical element can be a half wave plate. The first beam candiffract from the encoder scale twice. The first beam can be p-polarizedat the encoder scale.

In general, in another aspect, the invention features encoder systemsthat include a polarizing beam splitting element configured to reflect afirst beam in a first direction and transmit a second beam in a seconddirection orthogonal to the first direction and combine the first andsecond beams to form an output beam which exits the polarizing beamsplitting element along a path parallel to the second direction, whereinthe first and second beams have different frequencies and are derivedfrom a common source; a diffractive encoder scale positioned in the pathof the first beam so that the first beam diffracts from the diffractiveencoder scale at least once; a detector positioned to detect the outputbeam; and an electronic processor configured to receive an interferencesignal from the detector, the interference signal including a heterodynephase related to an optical path difference between the first and secondbeams, and determine the information about a degree of freedom of theencoder scale based on the heterodyne phase.

Embodiments of the encoder system can include one or more of thefollowing features and/or features of other aspects. For example, thediffractive encoder scale can be oriented orthogonal to the firstdirection. The first beam can be normally incident on the diffractiveencoder scale.

In general, in another aspect, the invention features methods fordetermining information about changes along a degree of freedom of agrating relative to an optical assembly, the methods including: using anoptical assembly to combine a first beam with a second beam to form anoutput beam, where the first beam is diffracted from a grating moveablewith respect to the optical assembly, the first and second beams arederived from a common source and the first beam is a non-zero diffractedorder of a primary beam derived from the common source that impinges onthe grating, the first beam being non-co-linear with the primary beam atthe grating; detecting an interference signal based on the output beam,the interference signal comprising a heterodyne phase related to anoptical path difference between the first beam and the second beam; anddetermining information about changes along a degree of freedom of thegrating relative to the optical assembly based on the heterodyne phase.

Implementations of the methods can include one or more of the followingfeatures and/or features of other aspects. For example, the primary beamcan be normally or non-normally incident on the grating.

The primary beam can be a linearly polarized beam. The primary beam canbe linearly polarized in a direction parallel to grating lines of thegrating prior to impinging on the grating.

The first and second beams can be polarized beams. The first beam can bepolarized orthogonal to the second beam. The first and second beams canbe linearly polarized beams.

In some embodiments, the second beam does not contact the grating.Alternatively, in certain embodiments, the second beam diffracts fromthe grating at least once. The second beam can be a zero-orderdiffracted beam. The second beam can be co-linear with the primary beamat the grating. The second beam can be a diffracted order of the primarybeam. The second beam can be the zeroth diffracted order of the primarybeam. The first beam can be a first diffracted order of the primarybeam. In some embodiments, the methods include combining a third beamand a fourth beam to form a second output beam, the third and fourthbeams being derived from the common source; and detecting a secondinterference signal based on the second output beam, the secondinterference signal comprising a heterodyne phase related to an opticalpath difference between the third beam and the fourth beam, wherein thethird beam is a beam diffracted from the grating, different from thefirst beam. The first beam and the third beam can be the +1 and −1diffracted orders of the primary beam, respectively. The second andfourth beams can be derived from zeroth diffracted order of the primarybeam.

The degree of freedom can correspond to a displacement of the gratingalong a first axis in a plane of the grating. The grating can includegrating lines extending along the first direction orthogonal to thefirst axis. The method can include determining information about asecond degree of freedom of the grating relative to the opticalassembly. The second degree of freedom can be a displacement of thegrating along a second axis orthogonal to the first axis. The secondaxis can be in the place of the grating. Alternatively, the second axiscan be orthogonal to the plane of the grating.

The degree of freedom can correspond to a displacement of the gratingalong an axis orthogonal to a plane of the grating.

The degree of freedom can be a tilt of the grating relative to theoptical assembly.

The methods can include monitoring a reference phase of an input beamproduced by the common source and from which the primary beam isderived. Determining the information can include comparing theheterodyne phase to the reference phase.

The first beam can be twice diffracted from the grating prior to beingcombined with the second beam. The first beam can be directed by aretroreflector to impinge on the grating at least once prior to beingcombined with the second beam.

The first and second beams can be different non-zeroth diffracted ordersof the primary beam.

The primary beam can include a first component and a second component,the first and second components having different frequencies that definea heterodyne frequency and different orthogonal polarization states. Theprimary beam can have a wavelength in a range from 400 nm to 1,500 nm.For example, the primary beam can have a wavelength of about 633 nm orabout 980 nm.

The grating can have a pitch in a range from about 1λ to about 20λ,where λ is a wavelength of the primary beam. The grating can have apitch in a range from about 1 μm to about 10 μam.

The methods can include translating the grating relative to the opticalassembly while determining the information. The optical assembly orgrating can be attached to a wafer stage and the method further includesmonitoring the position of a wafer relative to radiation from alithography system based on the information. In some embodiments, theoptical assembly or grating are attached to a reticle stage and themethod further includes monitoring the position of a reticle relative toradiation from a lithography system based on the information

In general, in another aspect, the invention features systems fordetermining information about changes along a degree of freedom of agrating relative to an optical assembly, the systems including: a lightsource configured to provide an input beam comprising a first componentand a second component, the first and second components having differentfrequencies and orthogonal polarization states; the optical assemblyconfigured to derive a primary beam from the input beam and direct theprimary beam to a grating, receive a first beam diffracted from thegrating at a non-zero order and combine it with a second beam to form anoutput beam, where the primary beam comprises the first component of theinput beam, the first beam is a diffracted order of the primary beam,the first beam being non-co-linear with the primary beam at the grating,the second beam comprises the second component of the input beam, andthe grating is moveable with respect to the optical assembly; a detectorpositioned to detect the output beam; and an electronic processorconfigured to receive an interference signal from the detector, theinterference signal comprising a heterodyne phase related to an opticalpath difference between the first beam and the second beam, anddetermine the information about the changes along the degree of freedomof the grating relative to the optical assembly based on the heterodynephase.

Embodiments of the systems can include one or more of the followingfeatures and/or features of other aspects. For example, the system caninclude a moveable stage and the grating is attached to the moveablestage.

The optical assembly can include a second grating positioned in the pathof the first beam. The second grating can be configured to redirect thefirst beam along a path parallel to a path of the primary beam. Theoptical assembly can include a retroreflector positioned in the path ofthe first beam to direct the first beam to contact the grating a secondtime. The optical assembly can include a polarizing beam splitterconfigured to derive the primary beam and second beam from the inputbeam. The optical assembly can include a polarizing beam splitterconfigured to combine the first and second beams to form the outputbeam. The optical assembly can be configured to direct the second beamto contact the grating.

In general, in another aspect, the invention features methods fordetermining information about changes along a degree of freedom of agrating relative to an optical assembly, the methods including: using anoptical assembly to combine a first beam with a second beam to form anoutput beam, where the first beam is diffracted from a grating moveablewith respect to the optical assembly, the first and second beams arederived from a common source and the first beam is a non-zero diffractedorder of a primary beam derived from the common source that impinges onthe grating, the first beam being non-co-linear with the primary beam atthe grating; detecting an interference signal based on the output beam,the interference signal comprising a phase related to an optical pathdifference between the first beam and the second beam; and determininginformation about changes along a degree of freedom of the gratingrelative to the optical assembly based on the phase, wherein the phaseis insensitive to first order to tip and/or tilt of the grating withrespect to the optical assembly.

Implementations of the methods can include one or more of the followingfeatures and/or features of other aspects. For example, the phase can bea heterodyne phase. The information can include information aboutchanges along at least two degrees of freedom of the grating relative tothe optical assembly. The at least two degrees of freedom can include adisplacement along an axis in a plane of the grating. The at least twodegrees of freedom can include a displacement along an axis orthogonalto the plane of the grating.

In general, in another aspect, the invention features systems formonitoring a displacement of a grating along an axis, the systemsincluding: a light source for providing illumination with two linearlypolarized components with different frequencies; a means for directingone or both components onto the grating; a means for receiving one orboth components diffracted from the grating; a means for combining andmixing both frequency components; a means for detecting an the combinedand mixed components; and a means for measuring a phase related to thedetected components, wherein one or both of the components do notsatisfy the Littrow condition at the grating. Embodiments of the systemcan include one or more features of other aspects.

Embodiments of aforementioned aspects can include one or more of thefollowing features. For example, the optical assembly can include a foldmirror arranged to direct the primary beam towards the grating. The foldmirror can direct the second beam towards the grating. The assembly caninclude a polarizing beam splitter arranged to split the input beam intothe primary beam and the second beam, wherein the primary and secondbeams lie within a first plane immediately after the beam splitter, andthe fold mirror is arranged to direct the primary and second beams in adirection orthogonal to the first plane. The primary and second beamscan contact the grating at a common location. The primary and secondbeams can contact the grating at the same incident angle but fromopposite sides of a normal to the grating. The first beam can beco-linear to a diffracted order of the second beam. The optical assemblycan include a first mirror and a second mirror configured torespectively reflect the primary and second beam towards the commonlocation. The beam splitter, fold mirror and first and second mirrorsmay all be interfaces of a monolithic optical element. The monolithicoptical element can have a maximum dimension orthogonal to the firstplane smaller than a maximum dimension of the monolithic optical elementin the first plane, e.g., the maximum dimension orthogonal to the firstplane can be 0.5 times or less (e.g., 0.3 times or less, 0.2 or less,0.1 times or less, 0.05 times or less) the maximum dimension in thefirst plane. The maximum dimension of the monolithic optical assemblyorthogonal to the first plane can be 5 cm or less (e.g., 4 cm or less, 3cm or less, 2 cm or less, 1 cm or less).

In another aspect, the invention features lithography methods for use infabricating integrated circuits on a substrate, the methods includingsupporting the substrate on a moveable stage, imaging spatiallypatterned radiation onto the substrate, adjusting the position of thestage, using the method or system of other aspects to monitor theposition of the stage, where the grating or the optical assembly areattached to the stage and the information corresponds to the position ofthe stage along an axis.

In another aspect, the invention features lithography methods forfabricating integrated circuits on a substrate including positioning afirst component of a lithography system relative to a second componentof a lithography system to expose the substrate to spatially patternedradiation and monitoring the position of the first component using themethod or system of other aspects, where the grating or the opticalassembly are attached to the first component and the informationcorresponds to the position of the first component.

In another aspect, the invention features lithography systems for use infabricating integrated circuits on a wafer, the systems including aprojection lens for imaging spatially patterned radiation onto thewafer, the system of another aspect configured to monitor the positionof the wafer relative to the imaged radiation, and a positioning systemfor adjusting the position of the stage relative to the imagedradiation, wherein the wafer is supported by the stage.

In another aspect, the invention features lithography systems for use infabricating integrated circuits on a wafer, the systems including anillumination system including a radiation source, a mask, a positioningsystem, a projection lens, and the system of another aspect, whereduring operation the source directs radiation through the mask toproduce spatially patterned radiation, the positioning system adjuststhe position of the mask relative to the radiation from the source, theprojection lens images the spatially patterned radiation onto the wafersupported by the stage, and the system monitors the position of the maskrelative to the radiation from the source.

Various references are incorporated herein by reference. In the event ofconflict, the present specification controls.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other features and advantages willbe apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an embodiment of an encoder system.

FIG. 2A is a schematic diagram of a portion of an embodiment of anencoder system.

FIG. 2B is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 3 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 4 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 5A is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 5B is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 5C is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 6A is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 6B is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 7 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 8 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 9 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 10 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 11 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 12 is a schematic diagram of a portion of another embodiment of anencoder system.

FIG. 13 is a schematic diagram of a portion of another embodiment of anencoder system.

FIGS. 14A, 14B, and 14C are schematic diagrams of another embodiment ofan encoder system. FIG. 14A shows an operational view of the encodersystem, FIG. 14B shows a top view, and FIG. 14C shows a side view.

FIGS. 15A, 15B, and 15C are schematic diagrams of another embodiment ofcomponents of an encoder system. FIG. 15A shows a top view of thecomponents, FIG. 15B shows a front view, and FIG. 15C shows a side view.

FIG. 16 is a schematic diagram of an embodiment of a lithography toolthat includes an interferometer.

FIG. 17A and FIG. 17B are flow charts that describe steps for makingintegrated circuits.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring to FIG. 1, an interferometric encoder system 100 includes alight source module 120 (e.g., including a laser), an optical assembly110, a measurement object 101, a detector module 130 (e.g., including apolarizer and a detector), and an electronic processor 150. Generally,light source module 120 includes a light source and can also includeother components such as beam shaping optics (e.g., light collimatingoptics), light guiding components (e.g., fiber optic waveguides) and/orpolarization management optics (e.g., polarizers and/or wave plates).Various embodiments of optical assembly 110 are described below. Theoptical assembly is also referred to as the “encoder head.” A Cartesiancoordinate system is shown for reference.

Measurement object 101 is positioned some nominal distance from opticalassembly 110 along the Z-axis. In many applications, such as where theencoder system is used to monitor the position of a wafer stage orreticle stage in a lithography tool, measurement object 101 is movedrelative to the optical assembly in the X- and/or Y-directions whileremaining nominally a constant distance from the optical assemblyrelative to the Z-axis. This constant distance can be relatively small(e.g., a few centimeters or less). However, in such applications, thelocation of measurement object typically will vary a small amount fromthe nominally constant distance and the relative orientation of themeasurement object within the Cartesian coordinate system can vary bysmall amounts too. During operation, encoder system 100 monitors one ormore of these degrees of freedom of measurement object 101 with respectto optical assembly 110, including a position of measurement object 101with respect to the x-axis, and further including, in certainembodiments, a position of the measurement object 101 with respect tothe y-axis and/or z-axis and/or with respect to pitch and yaw angularorientations.

To monitor the position of measurement object 101, source module 120directs an input beam 122 to optical assembly 110. Optical assembly 110derives a measurement beam 112 from input beam 122 and directsmeasurement beam 112 to measurement object 101. Optical assembly 110also derives a reference beam (not shown) from input beam 122 anddirects the reference beam along a path different from the measurementbeam. For example, optical assembly 110 can include a beam splitter thatsplits input beam 122 into measurement beam 112 and the reference beam.The measurement and reference beams can have orthogonal polarizations(e.g., orthogonal linear polarizations).

Measurement object 101 includes an encoder scale 105, which is ameasuring graduation that diffracts the measurement beam from theencoder head into one or more diffracted orders. In general, encoderscales can include a variety of different diffractive structures such asgratings or holographic diffractive structures. Examples or gratingsinclude sinusoidal, rectangular, or saw-tooth gratings. Gratings can becharacterized by a periodic structure having a constant pitch, but alsoby more complex periodic structures (e.g., chirped gratings). Ingeneral, the encoder scale can diffract the measurement beam into morethan one plane. For example, the encoder scale can be a two-dimensionalgrating that diffracts the measurement beam into diffracted orders inthe X-Z and Y-Z planes. The encoder scale extends in the X-Y plane overdistances that correspond to the range of the motion of measurementobject 110.

In the present embodiment, encoder scale 105 is a grating having gratinglines that extend orthogonal to the plane of the page, parallel to theY-axis of the Cartesian coordinate system shown in FIG. 1. The gratinglines are periodic along the X-axis. Encoder scale 105 has a gratingplane corresponding to the X-Y plane and the encoder scale diffractsmeasurement beam 112 into one or more diffracted orders in the Y-Zplane.

At least one of these diffracted orders of the measurement beam (labeledbeam 114), returns to optical assembly 110, where it is combined withthe reference beam to form an output beam 132. For example, theonce-diffracted measurement beam 114 can be the first-order diffractedbeam.

Output beam 132 includes phase information related to the optical pathlength difference between the measurement beam and the reference beam.Optical assembly 110 directs output beam 132 to detector module 130 thatdetects the output beam and sends a signal to electronic processor 150in response to the detected output beam. Electronic processor 150receives and analyzes the signal and determines information about one ormore degrees of freedom of measurement object 101 relative to opticalassembly 110.

In certain embodiments, the measurement and reference beams have a smalldifference in frequency (e.g., a difference in the kHz to MHz range) toproduce an interferometry signal of interest at a frequency generallycorresponding to this frequency difference. This frequency ishereinafter referred to interchangeably as the “heterodyne” frequency orthe “reference” frequency, and is denoted as ω_(R) (with respect toangular frequency). Information about the changes in the relativeposition of the measurement object generally corresponds to a phase ofthe interferometry signal at this heterodyne frequency. Signalprocessing techniques can be used to extract this phase. In general, themoveable measurement object causes this phase term to be time-varying.In this regard, the first order time derivative of the measurementobject movement causes the frequency of the interferometry signal toshift from the heterodyne frequency by an amount referred to herein asthe “Doppler” shift.

The different frequencies of the measurement and reference beams can beproduced, for example, by laser Zeeman splitting, by acousto-opticalmodulation, using two different laser modes, or internal to the laserusing birefringent elements, among other techniques. The orthogonalpolarizations allow a polarizing beam-splitter to direct the measurementand reference beams along different paths, and combine them to form theoutput beam that subsequently passes through a polarizer, which mixesthe orthogonally polarized components so they can interfere. In theabsence of target motion the interference signal oscillates at theheterodyne frequency, which is just the difference in the opticalfrequencies of the two components. In the presence of motion theheterodyne frequency incurs a change related to the velocity of thetarget through well-known Doppler relations. Accordingly, monitoringchanges in the heterodyne frequency allows one to monitor motion of thetarget relative to the optical assembly.

In the embodiments described below, the “input beam” generally, refersto the beam emitted by the light source module. For heterodynedetection, the input beam includes components having slightly differentfrequencies, as discussed above.

In general, the measurement beam is incident on measurement object 101at an incident angle such that the once-diffracted measurement beam doesnot satisfy the Littrow condition. The Littrow condition refers to anorientation of a diffractive structure, such as a grating, with respectto an incident beam where the diffractive structure directs thediffracted beam back towards the source. In other words, in encodersystem 100, the once-diffracted measurement beam is non-co-linear withthe measurement beam prior to diffracting at the encoder scale.

While encoder scale 105 is depicted in FIG. 1 as a structure that isperiodic in one direction, more generally, the measurement object caninclude a variety of different diffractive structures that appropriatelydiffract the measurement beam. In some embodiments, the measurementobject can include a diffractive structure (e.g., a encoder scale) thatis periodic in two directions (e.g., along the x- and y-axis),diffracting the measurement beam into beams in two orthogonal planes. Ingeneral, the diffractive structure of the encoder scale and sourcemodule are selected so that the encoder system provides one or morediffracted measurement beams having sufficient intensity to establishone or more detectable interference signals when combined withcorresponding reference beams, within the geometrical constraints forthe system. In some embodiments, the source module provides an inputbeam having a wavelength in a range from 400 nm to 1,500 nm. Forexample, the input beam can have a wavelength of about 633 nm or about980 nm. Note that, in general, the frequency splitting of the heterodynesource results in only a very small difference between the wavelength ofthe two components of the input beam, so even though the input beam isnot strictly monochromatic it remains practical to characterize theinput beam by a single wavelength. In some embodiments, the sourcemodule can include a gas laser (e.g., a HeNe laser), a laser diode orother solid-state laser source, a light-emitting diode, or a thermalsource such as a halogen light with or without a filter to modify thespectral bandwidth.

In general, the diffractive structure (e.g., grating pitch) can varydepending on the wavelength of the input beam and the arrangement ofoptical assembly and diffracted orders used for the measurement. In someembodiments, the diffractive structure is a grating having a pitch in arange from about 1λ to about 20λ, where λ is a wavelength of the source.The grating can have a pitch in a range from about 1 μm to about 10 μm.

Turning now to various implementations of encoder systems, in someembodiments, encoder systems are arranged so that the measurement beammakes a single pass to the encoder scale and a single diffracted orderof the measurement beam is used for the measurement. For example,referring to FIG. 2A, an optical assembly 110 of an encoder system 200includes a first polarizing beam splitter (PBS) 210, a second PBS 220,and a grating 211. Detector module 130 includes a polarizer 231 and adetector 230. PBS 210 splits input beam 122 into measurement beam 112and a reference beam 113. As shown, measurement beam 112 is polarized inthe plane of the figure (p-polarization), while secondary beam 113 ispolarized orthogonal to the plane of the figure (s-polarization).Measurement beam 112 is diffracted by encoder scale 105, providing aonce-diffracted measurement beam 114 that corresponds to a non-zerothdiffracted order (e.g., first order or second order) of measurement beam112. Grating 211, which can have a diffractive structure similar toencoder scale 105 (e.g., the same pitch) diffracts once-diffractedmeasurement beam 114 so that the now twice-diffracted measurement beamis incident on PBS 220 along a path parallel to the path of undiffractedmeasurement beam 112. PBS 220 combines twice-diffracted measurement beam114 with reference beam 113 to form output beam 132. At detector module130, polarizer 231 mixes the measurement and reference beam componentsof the output beam before the output beam is incident on detector 230.This can be achieved, for example, by orienting the transmission axis ofpolarizer 231 so that it transmits a component of s-polarized light anda component of p-polarized light (e.g., by orienting the transmissionaxis at 45° with respect to the plane of the page).

Encoder system 200 is an example of an encoder system that has a singledetection channel, where the measurement beam makes a single pass to themeasurement object. Here, the phase measured at detector 230 will varydepending on motion of encoder scale 105 in the X-direction and theZ-direction. Variations of this system are possible. For example, insome embodiments, measurement beam 114 can include both frequency andpolarization components. For example, referring to FIG. 2B, anon-polarizing beam splitter (NPBS) 212 can be used to split the inputbeam into the measurement beam and the reference beam so that bothcontain both s- and p-polarized light. However, once-diffractedmeasurement beam 114 and reference beam 113 are combined using PBS 220so that the only portion of once-diffracted measurement beam 114 inoutput beam 132 corresponds to the component polarized in one state (inthis case, p-polarized light), and the only portion of reference beam113 in output beam 132 corresponds to the component having theorthogonal polarization (here, s-polarized light).

Furthermore, in general, the optical assembly for encoder system 200 caninclude one or more components in addition, or as alternative, to thosecomponents shown in FIG. 2A. For example, in some embodiments,diffracted beam 114 can be redirected to PBS 220 using a refractivecomponent 213 (e.g., a prism or other refractive optical element)instead of grating 211. Such an embodiment is shown in FIG. 2B.

In certain embodiments, the encoder system includes additionalsubsystems. For example, in some embodiments, encoder system 200includes a local reference which monitors a phase of input beam 122. Asdepicted in FIG. 2A, a local reference can be provided using a beamsplitter 240 (e.g., a NPBS), polarizer 250, and a detector 260. Such areference can be useful, for example, in embodiments where the relativestarting phase between the components of input beam 122 is variable.

In some embodiments, encoder systems can provide more than onemeasurement channel. Additional channels can be provided by usingmultiple encoder heads. Alternatively, or additionally, in certainembodiments, a single encoder head can be configured to provide multiplemeasurement channels. For example FIG. 3 shows an encoder system 300that incorporates two measurement channels, each of which interfereeither the +1 or −1 measurement beam diffracted orders separately toboth improve motion sensitivity resolution and distinguish betweenencoder scale motion along the measurement beam axis (i.e., the Z-axis).Here, the second beam for each detection channel corresponds to thezeroth order diffraction of the measurement beam, which is nominallynormally incident on encoder scale 105.

Encoder system 300 includes NPBS 310 and NPBS 312, PBS 330 and PBS 332,gratings 320 and 322, polarizers 340 and 342, and detectors 350 and 352.Source module 120 directs input beam 122 through NPBS 310 and NPBS 312to contact encoder scale 105. Encoder scale 105 diffracts the incidentbeam into multiple orders, include the 0, +1, and −1 diffracted orders.The +1 diffracted beam is labeled as beam 321 and the −1 diffracted beamis labeled as beam 323. Each of these diffracted beams includespolarization components both in the plane of and orthogonal to thefigure.

The zeroth order diffracted beam travels back to NPBS 312 and NPBS 310.Beam splitter 312 splits this beam directing a portion as beam 353towards detector 352. Beam splitter 310 directs a portion as beam 351towards detector 350.

The +1 and −1 diffracted beams 321 and 323 propagate to gratings 320 and322, respectively. These gratings diffract beams 321 and 323 towardspolarizing beam splitters 330 and 332, respectively. Gratings 320 and322 can diffract the incident beams into additional directions, butother diffracted orders are omitted from the figures for clarity.

PBS 330 combines orthogonal polarization components from beam 321 and351 to provide a first output beam. Polarizer 340 is positioned in thepath of the output beam from polarizing beam splitter 330 and has itspass axis oriented to provide a mix of the orthogonal polarizationstates at detector 350. Similarly, PBS 332 combines orthogonalpolarization components from beam 323 and 353 to provide a second outputbeam. Polarizer 342 provides a mix of the orthogonal polarization statesin the second output beam to detector 352.

Since encoder scale motion in the Z direction is common to bothchannels' measurements, while encoder scale motion along X is detectedwith opposite signs, the two motions can be distinguished by a compositesignal consisting of the sum or difference of the two separate phases.For this case the basic equations for the change in phase as a functionof motion along X (Δx) and motion along Z (Δz) for the two detectors are

$\phi^{+} = {{{\frac{2\;\pi}{\Lambda}\Delta\; x} + {\frac{2\;\pi}{\lambda}\left( {1 + {\cos(\theta)}} \right)\Delta\; z\mspace{14mu}{and}\mspace{14mu}\phi^{-}}} = {{{- \frac{2\;\pi}{\Lambda}}\Delta\; x} + {\frac{2\;\pi}{\lambda}\left( {1 + {\cos(\theta)}} \right)\Delta\; z}}}$where the ± superscripts represent + or − order, λis the illuminationwavelength, Λ is the encoder scale period and the 1^(st) orderdiffraction angle (θ) is found from the encoder scale equation λ=Λsin(θ). To obtain the displacements in along the Z- and X-axes one formsthe sum and difference equations

${\Delta\; z} = {{\frac{\phi^{+} + \phi^{-}}{4\;\pi}\left( \frac{\lambda}{1 + {\cos(\theta)}} \right)\mspace{14mu}{and}\mspace{14mu}\Delta\; x} = {\frac{\phi^{+} - \phi^{-}}{4\;\pi}{\Lambda.}}}$

In some embodiments, additional channels can be provided for measurementof the displacements along the Y-axis. For such 2-dimensional (2D)applications (X and Y measurements) an area grating can be used. Forexample, encoder scale 105 can be periodic in both the X- andY-directions. The motion in the perpendicular (Y) axis can be obtainedwith another set of components rotated 90° about the Z-axis from thefirst, for example, providing two additional detection channels thatprovide a displacement in the Y-direction, Δy.

The embodiments shown in FIGS. 2A-2B and 3 suffer from sensitivity toboth in-plane and out-of-plane test encoder scale 105 angular motions(e.g., rotations about the y-axis). In certain embodiments, thissensitivity can be reduced. For example, this sensitivity can be reducedby adding a retroreflector as shown by the embodiment in FIG. 4 toprovide a double pass of measurement beam to encoder scale 105. Here,encoder system 400 includes a PBS 410 and retroreflectors 420 and 430.Source module 120 directs input beam 122 to PBS 410, which splits theinput beam into a measurement beam 112 and a reference beam 402, wheremeasurement beam 112 and reference beam 402 have orthogonal polarizationstates. Here, measurement beam 112 has s-polarization, which isreflected by the beam splitting interface of PBS 410, and reference beam402 has p-polarization, which is transmitted by the interface. PBS 410directs measurement beam 112 to encoder scale 105, which diffractsmeasurement beam 112 into various diffracted orders including aonce-diffracted measurement beam 401 (e.g., corresponding to the +1diffracted order). Retroreflector 430 is positioned to reflectonce-diffracted measurement beam 401 back towards encoder scale 105,which diffracts the measurement beam back towards PBS 410 parallel toundiffracted measurement beam 112. The polarization state of beam 401remains s-polarized, and is reflected by the interface of PBS 410towards detector module 130.

P-polarized reference beam 402 is reflected by retroreflector 420 backto PBS 410 and recombines with the s-polarized measurement beam nowtwice-diffracted from encoder scale 105 at the PBS interface. Theoverlapping s-polarized and p-polarized beams form an output beam, whichpropagates to detection module 130. The result of this is that rotationsof encoder scale 105 about the Y-axis result in a lateral displacementof twice-diffracted measurement beam relative to reference beam 402 whenthey are combined in the output beam, rather than an angular divergencebetween these beams.

In the same way as the embodiment shown in FIG. 3 implements a designsimilar to the encoder system in FIG. 2A adapted to monitor two channelsbased on the +/−1 diffracted orders, respectively, encoder system 400 inFIG. 4 can be adapted to utilize both the +/−1 diffracted orders formultiple measurement channels. For example, encoder system 500, shown inFIGS. 5A and 5B, provides two measurement channels which can be used forZ-motion discrimination. FIG. 5A shows encoder system 500 in the X-Zplane, while FIG. 5B shows the system in the Y-Z plane. Encoder system500 includes a beam-splitting prism 510, a PBS 520 and retroreflectors530, 540, and 550. Beam splitting prism 510 splits the input beam fromsource module 120 into two parallel beams 522 and 524 displaced fromeach other, which are then each split into orthogonally polarized beamsat the beam splitting interface of PBS 520. PBS 520 directs one pair ofbeams 525 and 526 to encoder scale 105 and transmits the other pair ofbeams 527 and 528 to retroreflector 530.

Retroreflector 540 is positioned to retroreflect +1 order diffractedlight from beam 525. Similarly, retroreflector 550 is positioned toretroreflect −1 order diffracted light from beam 526. The retroreflectedbeams from each retroreflector both diffract at encoder scale 105 asecond time and the twice-diffracted beams are directed back towards PBS520.

Beam blocks 541 and 551 are provided at retroreflectors 540 and 550,respectively, in the once-diffracted measurement beam path to isolatethe appropriate diffracted order for each of the encoder scaleinteractions. Specifically, beam block 541 is positioned atretroreflector 540 to block +1 diffracted order light from beam 526.Beam block 551 is positioned at retroreflector 550 to block −1diffracted order light from beam 525. The blocked beams are illustratedby dashed lines in FIGS. 5A and 5B.

The beams directed back to PBS 520 from encoder scale 105 overlap withbeams 527 and 528 at the interface of PBS 520 to provide a pair ofoutput beams 531 and 532. Detector module 130 includes a pair ofdetectors 535 and 536 positioned to detect output beams 531 and 532,respectively. Detector module also includes polarizers (not shown) tomix the orthogonally polarized components of the output beams.

In encoder system 500, the measurement beams are s-polarized. However,in general, encoder systems can be configured so that the measurementbeam has other polarization states. For example, the polarization stateof the measurement beam can be chosen to provide maximum diffractionintensity from encoder scale 105 into the diffracted order used for themeasurement beam. For example, encoder systems can be arranged to sothat the measurement beam has p-polarization.

Referring to FIG. 5C, an encoder system 1700, laid out in threedimensions, is configured to provide two measurement channels where themeasurement beams have p-polarization at a grating 1701. Encoder system1700 has an encoder head that includes a NPBS 1750, a PBS 1710 andretroreflectors 1730, 1740, and 1750. Encoder system 1700 also includesa half wave plate 1711 positioned at the face of PBS 1710 that facesgrating 1701.

NPBS 1750 splits input beam into two beams and directs them alongparallel paths to PBS 1710. PBS 1710 splits each of these beams into ameasurement beam and a reference beam. The measurement beam hass-polarization and the reference beam has p-polarization. The referencebeams reflect from retroreflector 1720 back to PBS 1710. The measurementbeams pass through half wave plate 1711 which converts them froms-polarization to p-polarization. Both diffract at grating 1701.Retroreflector 1730 is positioned to reflect the +1 diffracted order ofa first of the measurement beams. Retroreflector 1740 reflects the −1diffracted order of the other measurement beam.

The measurement beams diffract from grating 1701 a second time back toPBS 1710. Prior to entering the PBS cube, half wave plate 1711 convertsthe polarization state of the measurement beams back from p-polarizationto s-polarization. The measurement beams recombine with respectivereference beams to form two output beams 1760 and 1770, which exit PBS1710 propagating parallel to the Y-axis.

In encoder system 1700, the fact that the measurement beams ares-polarized with respect to the PBS, but have p- polarization withrespect to the diffracted beams of the encoder scale can lead to higherdiffraction efficiency and more efficient overall use of the input beamenergy. Furthermore the encoder head can be relatively compact,particularly in the Z-direction. For example, in some embodiments, theencoder head can fit within a volume of less than 5 cubic inches (e.g.,about 3 cubic inches or less, about 2 cubic inches or less, about 1cubic inch or less). In certain embodiments, the encoder head has athickness of an inch or less in the Z-direction (e.g., 1 cm or less, 0.5cm or less).

In some embodiments, polarization modifying elements (e.g., wave platesof various retardations, such as λ/4 and/or λ/8 plates) can beincorporated a way to shift one or more of the beams to more convenientdetection locations. For example, quarter wave plates can be positionedin the path of one of the diffracted measurement beams and acorresponding one of the reference beams, resulting in the correspondingoutput beam exiting the PBS from a different face than the other outputbeam. FIG. 6A shows an encoder system 600 similar in construction toencoder system 500 except that encoder system 600 includes quarter waveplates 552 and 553. Quarter wave plate 552 is positioned in the path ofmeasurement beam 525 diffracted from encoder scale 105 and quarter waveplate 553 is positioned between PBS 520 and retroreflector 530 in thepath of reference beam 528. Both beams make a double pass through therespective quarter wave plates so the effect of the wave plates is totransform the polarization state of the beam by rotating it through 900.This change in polarization results in output beam 532 exiting PBS 520through a different face than output beam 531.

As another example, referring to FIG. 6B, in encoder system 600′, a λ/8plate 554 is positioned between PBS 520 and retroreflector 530 in thepath of both beams 527 and 528 to provide a heterodyne reference at bothpolarization states. In this embodiment, output beams 531 and 532 exitPBS 520 out different faces. λ/8 plate 554 in conjunction with PBS 520split the reference beam toward both faces of the cube, allowing accessto the interference signal at a face that depends only on thepolarization state of the test beams. However, more generally, outputbeams can exit out of either face depending on which measurement beampaths include a quarter wave plate. For example, placing a λ/4 platepositioned to intercept the two returning test beams at the top face ofthe PBS would allow convenient access to the interference signal at bothfaces.

In general, the use of polarization modulating elements can also enablethe option of having collinear and coextensive beams for the left andright-hand retroreflectors, as shown, for example, in FIG. 7. Here, anencoder system 700 separates the output beams based on polarizationstate, rather than using spatial separation as before to provide twochannel measurements. Encoder system 700 is similar to system 600 in thearrangement of PBS 520 and retroreflectors 530, 540, and 550. However,encoder system 700 does not include a beam splitter between sourcemodule 120 and PBS 520. PBS 520 splits input beam 122 into ans-polarized beam 722 and a p-polarized beam 721. Beam 722 diffracts fromencoder scale 105 into multiple diffracted orders including the +1diffracted order (beam 723) and the −1 diffracted order (beam 724).Retroreflectors 540 and 550 are positioned to reflect beams 723 and 724,respectively. Polarizers 810 and 812 are positioned in the path of theonce-diffracted measurement beams 724 and 723 before they are reflectedby the retroreflectors. Both polarizers have their transmission axesoriented to transmit s-polarized light. The polarizers limit therecirculation of the beam between retroreflectors 540 and 550. Forexample, in the absence of polarizer 810, consider beam 723 aftertravelling through retroreflector 540 and striking encoder scale 105.There will be a 0^(th) order reflection that passes throughretroreflector 550, reflects from encoder scale 105 again andrecirculates though retroreflector 540 before diffracting from encoderscale 105 to become beam 725. Polarizers 810 and 812 eliminate thisrecirculating beam and its counterpart that traverses the retroreflector540 first.

Encoder system 700 also includes a quarter wave plate 801 betweenretroreflector 550 and encoder scale 105 in the path of beam 724. Thedouble pass of beam 724 through quarter wave plate 801 transforms thepolarization state of beam 724 from s-polarization to p-polarization.Beams 723 and 724 are incident at the same location of encoder scale 105where they are diffracted again by the encoder scale. Twice-diffractedbeams 723 and 724 recombine to form beam 725 that propagates backtowards PBS 520 along a path parallel to beam 722.

Encoder system 700 includes a λ/8 plate 532 between PBS 520 andretroreflector 730. Reference beam 721, which is reflected byretroreflector 530, passes through λ/8 plate twice and is transformedfrom p-polarized light to circularly polarized light. PBS 520 combinesthe s-polarized component of measurement beam 725 and the p-polarizedcomponent of reference beam 721 to form a first output beam 558 that isdetected by detector 560. PBS 520 also combines the p-polarizedcomponent of measurement beam 725 with the s-polarized component ofreference beam 721 to form a second output beam 559 that is detected bydetector 561. Polarizers can be positioned between PBS 520 and detectors560 and 561, but are not shown in FIG. 7.

In general, while the foregoing embodiments utilize the zeroth and/or+/−1^(st) order diffraction from the encoder scale, higher diffractionorders can also be used. For example, referring to FIG. 8, in someembodiments, one or both components of the input beam are directed toencoder scale 105 and one diffracted component is allowed to interferewith another diffracted component. FIG. 8 shows an encoder system 800that includes a mirror 830 (or other reflective element), a grating 840,and a polarizing beam splitter 810. Encoder system 800 further includessource module 120 and a detector module including a detector 820 and apolarizer 812.

Source module 120 illuminates encoder scale 105 with input beam 122which is diffracted into multiple orders including the +1 diffractedorder (beam 821) and +2 diffracted order (beam 822). Beam 821 propagatesto PBS 810 via grating 840 and beam 822 propagates to beam 822 viamirror 830. PBS 810 combines the s-polarized component of beam 821 andthe p-polarized component of beam 822 to form an output beam that isdirected through polarizer 812 to detector 820.

In encoder system 800, the 1^(st) and 2^(nd) order diffracted beamsinterfere. One advantage in allowing both interfering components tointeract with the encoder scale is that out-of-plane encoder scalerotations (e.g., about the Y-axis) are then common mode because both thefirst and second diffraction orders are subject to the same rotation. Asimilar embodiment is shown in FIG. 9. Here, encoder system 900 uses abulk optic component 850 (e.g., a prism) instead of the grating 840 usedin encoder system 800. In addition, the system is configured to measurean output beam formed from the p-polarized component of beam 821 and thes-polarized component of beam 822.

More complicated structures can be built up to, e.g., to provideadditional measurement channels and/or improve sensitivity and orminimize sources of error. For example, FIG. 10 shows a two-measurementchannel encoder system 1000 that uses the +1, +2 and −1, −2 diffractedorders separately to both improve motion sensitivity resolution anddistinguish between encoder scale motion along the measurement beam axis(Z-axis). For one measurement channel, encoder system 1000 uses the samestructure as encoder system 900. The second channel is provided using anidentical set of components (i.e., a PBS 1010, a bulk optical component1050, a mirror 1030, a polarizer 1012, and a detector 1020) arrangedsymmetrically with respect to input beam 122 to utilize the −1 and −2diffracted orders.

Since encoder scale motion in the Z direction is common to bothchannels, while encoder scale motion along X is detected with oppositesigns, the two motions can be distinguished by a composite signalconsisting of the sum or difference of the two separate phases. For theembodiment shown in FIG. 10, the basic equations for the change in phaseas a function of motion along X (Δx) and motion along Z (Δz) for the twodetectors are

$\begin{matrix}{\phi^{+} = {{\frac{2\;\pi}{\lambda}\left( {1 + {\cos\left( \theta_{2} \right)}} \right)\Delta\; z} + {2\frac{2\;\pi}{\Lambda}\Delta\; x} - \left\lbrack {{\frac{2\;\pi}{\lambda}\left( {1 + {\cos\left( \theta_{1} \right)}} \right)\Delta\; z} + {\frac{2\;\pi}{\Lambda}\Delta\; x}} \right\rbrack}} \\{= {\frac{2\;\pi}{\Lambda}\left\lbrack {{\Delta\; x} + {\Delta\;{z\left( {{\cos\left( \theta_{2} \right)} - {\cos\left( \theta_{1} \right)}} \right)}}} \right\rbrack}} \\{= {\frac{2\;\pi}{\Lambda}\left\lbrack {{\Delta\; x} + {\Delta\;{z\left( {\sqrt{1 - \left( \frac{2\;\lambda}{\Lambda} \right)^{2}} - \sqrt{1 - \left( \frac{\lambda}{\Lambda} \right)^{2}}} \right)}}} \right\rbrack}}\end{matrix}$ $\begin{matrix}{\phi^{-} = {{\frac{2\;\pi}{\lambda}\left( {1 + {\cos\left( \theta_{2} \right)}} \right)\Delta\; z} - {2\frac{2\;\pi}{\Lambda}\Delta\; x} - \left\lbrack {{\frac{2\;\pi}{\lambda}\left( {1 + {\cos\left( \theta_{1} \right)}} \right)\Delta\; z} + {\frac{2\;\pi}{\Lambda}\Delta\; x}} \right\rbrack}} \\{= {\frac{2\;\pi}{\Lambda}\left\lbrack {{{- \Delta}\; x} + {\Delta\;{z\left( {{\cos\left( \theta_{2} \right)} - {\cos\left( \theta_{1} \right)}} \right)}}} \right\rbrack}} \\{= {\frac{2\;\pi}{\Lambda}\left\lbrack {{{- \Delta}\; x} + {\Delta\;{z\left( {\sqrt{1 - \left( \frac{2\;\lambda}{\Lambda} \right)^{2}} - \sqrt{1 - \left( \frac{\lambda}{\Lambda} \right)^{2}}} \right)}}} \right\rbrack}}\end{matrix}$where the ±superscripts represent +or − order, λ is the illuminationwavelength, Λ is the encoder scale period and the encoder scale equationnλ=Λ sin(θ_(n)) describes the n^(th) order diffraction angle. To obtainthe displacements one forms the following simple sum and differenceequations

${\Delta\; z} = {{\frac{\phi^{+} + \phi^{-}}{4\;\pi}\frac{\Lambda}{\left( {\sqrt{1 - \left( \frac{2\;\lambda}{\Lambda} \right)^{2}} - \sqrt{1 - \left( \frac{\lambda}{\Lambda} \right)^{2}}} \right)}\mspace{14mu}{and}\mspace{14mu}\Delta\; x} = {\frac{\phi^{+} - \phi^{-}}{4\;\pi}{\Lambda.}}}$

In general, embodiments can feature more than two measurement channelsand/or can be arranged to measure tilt angles of the measurement objectin addition or alternative to encoder scale displacements. For example,with reference to FIG. 11, an encoder system 1100 allows the calculationof measurement object local tilt by splitting input beam 122 andinterrogating encoder scale 105 at two separated points. In the presentexample, encoder system 1100 includes a grating 1110 (e.g., having thesame pitch as encoder scale 105) which splits input beam 122 into twobeams 1101 and 1102 (e.g., the −1 and +1 diffracted orders,respectively). Other types of beam splitter, such as a non-diffractivebeam splitter, can be used.

A first measurement channel is provided from the +1 and zerothdiffracted orders of beam 1102. Encoder system 1100 includes a PBS 1114positioned in the path of the +1 diffracted order and a prism 1124 and aNPBS 1134 in the path of the zeroth diffracted order. NPBS 1134 directsa portion of the zeroth diffracted order of beam 1102 to PBS 1114 whereit is combined with a component of the +1 diffracted order to form afirst output beam 1152, which is analyzed by a polarizer 1151 beforedetection by a detector 1150. The interference phase detected at thisdetector is sensitive to X-motion of encoder scale 105 and to Z-motionat the location where beam 1102 strikes encoder scale 105.

A second measurement channel is provided from the −1 and zerothdiffracted orders of beam 1101. Encoder system 1100 includes apolarizing beam splitter 1112 positioned in the path of the −1diffracted order and a prism 1122 and non-polarizing beam splitter 1132in the path of the zeroth diffracted order. Beam splitter 1132 directs aportion of the zeroth diffracted order of beam 1101 to PBS 1112 where itis combined with a component of the −1 diffracted order to form a secondoutput beam 1142. A polarizer 1141 analyzes output beam 1142, which isthen detected by a detector 1140. The interference phase detected atdetector 1140 is sensitive to X-motion of encoder scale 105 and toZ-motion at the location where beam 1101 strikes encoder scale 105.

A third measurement channel is provided using zeroth diffracted order ofbeam 1101 and the zeroth diffracted order of beam 1102. Encoder system1100 includes an optical element (e.g., a prism) which reflects aportion of the zeroth diffracted order of beam 1101 towards a polarizingbeam splitter 1144 positioned adjacent to non-polarizing beam splitter1134, where it is combined with a portion of the zeroth diffracted orderof beam 1102. The combined beam is detected by detector 1160.

The third measurement channel, which effectively monitors themeasurement object tilt about the geometric mean between the two points,is necessary to solve for the separate motions since they are coupled.The equations governing the phase measurements in this geometry are

$\phi^{+} = {{\frac{2\;\pi}{\lambda}\left( {\frac{1}{\cos(\theta)} - 1} \right)\Delta\; z^{+}} + {\frac{2\;\pi}{\Lambda}\Delta\; x}}$$\phi^{-} = {{\frac{2\;\pi}{\lambda}\left( {\frac{1}{\cos(\theta)} - 1} \right)\Delta\; z^{-}} - {\frac{2\;\pi}{\Lambda}\Delta\; x}}$$\phi^{z} = {\frac{2\;\pi}{\lambda}\left( \frac{2}{\cos(\theta)} \right)\left( {{\Delta\; z^{+}} - {\Delta\; z^{-}}} \right)}$where 0 is the 1^(st) order diffraction angle given by λ=Λ sin(θ), φ^(±)are the two phases from the +/− diffracted beams and φ^(z) is the phasefrom the interference of the two 0^(th) order beams. This is a simplelinear system of three equations. Solving for the three motions;

${\Delta\; z^{+}} = {\frac{\lambda}{4\;\pi}\left\lbrack {{\left( {\phi^{+} + \phi^{-}} \right)\left( \frac{\cos(\theta)}{1 - {\cos(\theta)}} \right)} + {\phi^{z}\frac{\cos(\theta)}{2}}} \right\rbrack}$${\Delta\; z^{-}} = {\frac{\lambda}{4\;\pi}\left\lbrack {{\left( {\phi^{+} + \phi^{-}} \right)\left( \frac{\cos(\theta)}{1 - {\cos(\theta)}} \right)} - {\phi^{z}\frac{\cos(\theta)}{2}}} \right\rbrack}$${\Delta\; x} = {\frac{\Lambda}{4\;\pi}\left\lbrack {\phi^{+} - \phi^{-} - {\phi^{z}\left( \frac{1 - {\cos(\theta)}}{2} \right)}} \right\rbrack}$Duplicating this geometry in the YZ plane provides motion monitoringalong both X- and Y-axes.

Another embodiment is shown in FIG. 12 where two frequency componentsare separated in space and angle allowing combination of differentdiffracted orders automatically upon interacting with encoder scale 105.Here, encoder 1200 includes a PBS 1210, which splits input beam 122 intoan s-polarized beam 1222 and a p-polarized beam 1224. PBS 1210 directsbeam 1222 towards encoder scale 105 and transmits beam 1224, whichreflects from a mirror 1220 towards encoder scale 105. Mirror 1220 isoriented so that beam 1224 contacts encoder scale 105 at the samelocation as beam 1222. Beam 1222 is normally (at least nominally)incident on encoder scale 105 and beam 1224 is incident on encoder scale105 along a non-normal direction. Both beams are diffracted intomultiple diffracted orders by encoder scale 105 and the system isarranged so that two diffracted orders from each of beams 1222 and 1224overlap, providing two output beams 1231 and 1233. While, in general, avariety of different diffracted orders can be selected for the outputbeams, in some embodiments, output beam 1231 can be formed from the −1diffracted order of beam 1222 and the +3 diffracted order of beam 1224.Output beam 1233 can be formed from the +1 diffracted order of beam 1222and the +1 diffracted order of beam 1224. Detectors 1230 and 1232 arepositioned to detect the output beams.

FIG. 13 shows a further embodiment, encoder system 1300, similar toencoder system 1200 but where the beam geometry is accomplished throughthe use of two fixed gratings, 1305 and 1310 instead of a PBS. Encodersystem 1300 also includes two polarizers 1312 and 1314, positionedbetween grating 1305 and 1310. Here, grating 1305 diffracts input beam122 into various diffracted orders including beams 1322 and 1324 (e.g.,the +1 and −1 diffracted orders). Polarizer 1312 is positioned in thepath of beam 1322 and transmitted the s-polarized component of beam1322. Polarizer 1314 is positioned in the path of beam 1324 andtransmits the p-polarized component of this beam. Now-polarized beams1322 and 1324 are diffracted by grating 1310 and propagate towardsencoder scale 105 along paths corresponding to beams 1222 and 1224,respectively, as described with reference to FIG. 12.

Encoder systems 1200 and 1300 show different optical arrangements forachieving similar measurements. Other configurations, includingcombinations of diffractive and/or non-diffractive optics, are possible.

In general, the path of one or more beams used in an encoder system canbe adapted to spatial requirements of its end-use application. Forexample, one or more beam paths can be folded to conform to the encodersystem to a specific space. For example, referring to FIGS. 14A-14C, anencoder system 1400 includes a PBS 1410 and a pair of mirrors 1422 and1424 that lie in a plane (the X-Y plane as shown in FIG. 16A) parallelto the plane in which encoder scale 105 lies. FIG. 14A shows anoperational view of encoder system 1400, while FIGS. 14B and 14C showviews in the X-Y and Y-Z planes, respectively. PBS 1410 splits lightfrom the source into two beams 1421 and 1423 having orthogonalpolarization states, and each reflects from one of mirrors 1422 and 1424so that the beams are redirected to contact encoder scale 105 at acommon point. A fold mirror 1412 redirects beams 1421 and 1423 out ofthe X-Y plane towards encoder scale 105, which diffracts the incidentlight into one or more diffracted orders including a pair of parallel,co-extensive beams that form output beam 1401, leaving encoder scale 105parallel to the Z-axis. A detector 1420 is positioned to receive outputbeam 1401, providing a heterodyne interference signal in the same manneras discussed above.

As shown in FIG. 14A, beams 1421 and 1423 from the PBS 1410 are incidenton encoder scale 105 at an incident angle, θ, measured with respect tothe Z-axis. Mirror 1412 includes an aperture 1415 positioned in the pathof output beam 1401, allowing the output beam passage through todetector 1420.

Of course, other configurations for directing the diffracted beams tothe detector are also possible. For example, in some embodiments, a beamsplitter can be used rather than a mirror, allowing passage of a portionof the diffracted light to the detector. Alternatively, or additionally,an additional mirror can be placed between fold mirror 1412 and PBS1410, oriented to direct output beam 1401 to the detector.

Encoder system 1400 may have a number of advantages. For example, theencoder system has, to 1^(st) order, no tilt or yaw sensitivity but isonly sensitive to X motion (as shown). Further, such encoder systems canprovide relatively efficient use of light from the source, for example,relative to encoder systems where the light makes multiple passes to themeasurement object, in each of which only a fraction of the incidentlight is diffracted into the usable order(s).

In embodiments, folding the optical path of the encoder system can allowa designer to adapt the encoder head optics to spaces that may berelatively narrow in at least one dimension. For example, the encodershown in FIGS. 16A-16C can have a small footprint in the Z-direction,allowing such encoders to be installed in relatively small spaces in theZ-direction.

In embodiments where sensitivity to additional degrees of freedom of themeasurement object is desired, additional encoder heads or other devicescan be provided. For example, for Y motion sensitivity, another encoderhead, similar to the one shown in FIGS. 14A-14C, oriented at 90° (aboutthe Z-axis) may be included. Of course, such a configuration wouldinclude the measurement object to diffract light in the Y-direction aswell as the X-direction. For example, the measurement object can includetwo gratings oriented at right angles, or can be periodic in both the X-and Y-directions. For Z-motion, a displacement measuring interferometer(e.g., a high stability plane mirror interferometer, striking theencoder scale normally) can be used.

In certain embodiments, X and Y motion encoders and a Z-motioninterferometer can sample different points on the measurement object,but without Z sensitivity sampling different points generally does notintroduce tilt sensitivity.

In certain embodiments, folding optics (e.g., fold mirrors) can beintegrated with other components of the optical assembly forming theencoder head. For example, referring to FIGS. 15A-15C, an encoder system1500 includes a compound monolithic assembly 1501 in which a PBSinterface 1510, mirrors 1512 and 1514, and a fold mirror 1520 are allprovided by different interfaces of a single compound optical component.For example, assembly 1501 can be formed from two pieces of, e.g.,glass, glued together with a PBS coating at the glue interface. Mirrorinterfaces 1512 and 1514 can include reflective coatings (e.g., silveror multilayer dielectric coatings), or can be arranged so that thereflection occurs due to total internal reflection within the element.The assembly can have a relatively small dimension in the Z-direction.For example, the assembly can be less than 1 inch thick in theZ-direction (e.g., about 1 cm or less, about 0.5 cm or less). Dependingon the end use, the assembly can occupy a volume of 1 cubic inch orless.

In general, any of the analysis methods described above, includingdetermining information about a degree of freedom of the encoder scales,can be implemented in computer hardware or software, or a combination ofboth. For example, in some embodiments, electronic processor 150 can beinstalled in a computer and connected to one or more encoder systems andconfigured to perform analysis of signals from the encoder systems.Analysis can be implemented in computer programs using standardprogramming techniques following the methods described herein. Programcode is applied to input data (e.g., interferometric phase information)to perform the functions described herein and generate outputinformation (e.g., degree of freedom information). The outputinformation is applied to one or more output devices such as a displaymonitor. Each program may be implemented in a high level procedural orobject oriented programming language to communicate with a computersystem. However, the programs can be implemented in assembly or machinelanguage, if desired. In any case, the language can be a compiled orinterpreted language. Moreover, the program can run on dedicatedintegrated circuits preprogrammed for that purpose.

Each such computer program is preferably stored on a storage medium ordevice (e.g., ROM or magnetic diskette) readable by a general or specialpurpose programmable computer, for configuring and operating thecomputer when the storage media or device is read by the computer toperform the procedures described herein. The computer program can alsoreside in cache or main memory during program execution. The analysismethods can also be implemented as a computer-readable storage medium,configured with a computer program, where the storage medium soconfigured causes a computer to operate in a specific and predefinedmanner to perform the functions described herein.

Lithography Tool Applications

Lithography tools are especially useful in lithography applications usedin fabricating large scale integrated circuits such as computer chipsand the like. Lithography is the key technology driver for thesemiconductor manufacturing industry. Overlay improvement is one of thefive most difficult challenges down to and below 100 nm line widths(design rules), see, for example, the Semiconductor Industry Roadmap, p.82 (1997).

Overlay depends directly on the performance, i.e., accuracy andprecision, of the metrology system used to position the wafer andreticle (or mask) stages. Since a lithography tool may produce$50-100M/year of product, the economic value from improved metrologysystems is substantial. Each 1% increase in yield of the lithographytool results in approximately $1M/year economic benefit to theintegrated circuit manufacturer and substantial competitive advantage tothe lithography tool vendor.

The function of a lithography tool is to direct spatially patternedradiation onto a photoresist-coated wafer. The process involvesdetermining which location of the wafer is to receive the radiation(alignment) and applying the radiation to the photoresist at thatlocation (exposure).

During exposure, a radiation source illuminates a patterned reticle,which scatters the radiation to produce the spatially patternedradiation. The reticle is also referred to as a mask, and these termsare used interchangeably below. In the case of reduction lithography, areduction lens collects the scattered radiation and forms a reducedimage of the reticle pattern. Alternatively, in the case of proximityprinting, the scattered radiation propagates a small distance (typicallyon the order of microns) before contacting the wafer to produce a 1:1image of the reticle pattern. The radiation initiates photo-chemicalprocesses in the resist that convert the radiation pattern into a latentimage within the resist.

To properly position the wafer, the wafer includes alignment marks onthe wafer that can be measured by dedicated sensors. The measuredpositions of the alignment marks define the location of the wafer withinthe tool. This information, along with a specification of the desiredpatterning of the wafer surface, guides the alignment of the waferrelative to the spatially patterned radiation. Based on suchinformation, a translatable stage supporting the photoresist-coatedwafer moves the wafer such that the radiation will expose the correctlocation of the wafer. In certain lithography tools, e.g., lithographyscanners, the mask is also positioned on a translatable stage that ismoved in concert with the wafer during exposure.

Encoder systems, such as those discussed previously, are importantcomponents of the positioning mechanisms that control the position ofthe wafer and reticle, and register the reticle image on the wafer. Ifsuch encoder systems include the features described above, the accuracyof distances measured by the systems can be increased and/or maintainedover longer periods without offline maintenance, resulting in higherthroughput due to increased yields and less tool downtime.

In general, the lithography tool, also referred to as an exposuresystem, typically includes an illumination system and a waferpositioning system. The illumination system includes a radiation sourcefor providing radiation such as ultraviolet, visible, x-ray, electron,or ion radiation, and a reticle or mask for imparting the pattern to theradiation, thereby generating the spatially patterned radiation. Inaddition, for the case of reduction lithography, the illumination systemcan include a lens assembly for imaging the spatially patternedradiation onto the wafer. The imaged radiation exposes resist coatedonto the wafer. The illumination system also includes a mask stage forsupporting the mask and a positioning system for adjusting the positionof the mask stage relative to the radiation directed through the mask.The wafer positioning system includes a wafer stage for supporting thewafer and a positioning system for adjusting the position of the waferstage relative to the imaged radiation. Fabrication of integratedcircuits can include multiple exposing steps. For a general reference onlithography, see, for example, J. R. Sheats and B. W. Smith, inMicrolithography: Science and Technology (Marcel Dekker, Inc., New York,1998), the contents of which is incorporated herein by reference.

Encoder systems described above can be used to precisely measure thepositions of each of the wafer stage and mask stage relative to othercomponents of the exposure system, such as the lens assembly, radiationsource, or support structure. In such cases, the encoder system'soptical assembly can be attached to a stationary structure and theencoder scale attached to a movable element such as one of the mask andwafer stages. Alternatively, the situation can be reversed, with theoptical assembly attached to a movable object and the encoder scaleattached to a stationary object.

More generally, such encoder systems can be used to measure the positionof any one component of the exposure system relative to any othercomponent of the exposure system, in which the optical assembly isattached to, or supported by, one of the components and the encoderscale is attached, or is supported by the other of the components.

An example of a lithography tool 1800 using an interferometry system1826 is shown in FIG. 16. The encoder system is used to preciselymeasure the position of a wafer (not shown) within an exposure system.Here, stage 1822 is used to position and support the wafer relative toan exposure station. Scanner 1800 includes a frame 1802, which carriesother support structures and various components carried on thosestructures. An exposure base 1804 has mounted on top of it a lenshousing 1806 atop of which is mounted a reticle or mask stage 1816,which is used to support a reticle or mask. A positioning system forpositioning the mask relative to the exposure station is indicatedschematically by element 1817. Positioning system 1817 can include,e.g., piezoelectric transducer elements and corresponding controlelectronics. Although, it is not included in this described embodiment,one or more of the encoder systems described above can also be used toprecisely measure the position of the mask stage as well as othermoveable elements whose position must be accurately monitored inprocesses for fabricating lithographic structures (see supra Sheats andSmith Microlithography: Science and Technology).

Suspended below exposure base 1804 is a support base 1813 that carrieswafer stage 1822. Stage 1822 includes a measurement object 1828 fordiffracting a measurement beam 1854 directed to the stage by opticalassembly 1826. A positioning system for positioning stage 1822 relativeto optical assembly 1826 is indicated schematically by element 1819.Positioning system 1819 can include, e.g., piezoelectric transducerelements and corresponding control electronics. The measurement objectdiffracts the measurement beam reflects back to the optical assembly,which is mounted on exposure base 1104. The encoder system can be any ofthe embodiments described previously.

During operation, a radiation beam 1810, e.g., an ultraviolet (UV) beamfrom a UV laser (not shown), passes through a beam shaping opticsassembly 1812 and travels downward after reflecting from mirror 1814.Thereafter, the radiation beam passes through a mask (not shown) carriedby mask stage 1816. The mask (not shown) is imaged onto a wafer (notshown) on wafer stage 1822 via a lens assembly 1808 carried in a lenshousing 1806. Base 1804 and the various components supported by it areisolated from environmental vibrations by a damping system depicted byspring 1820.

In some embodiments, one or more of the encoder systems describedpreviously can be used to measure displacement along multiple axes andangles associated for example with, but not limited to, the wafer andreticle (or mask) stages. Also, rather than a UV laser beam, other beamscan be used to expose the wafer including, e.g., x-ray beams, electronbeams, ion beams, and visible optical beams.

In certain embodiments, the optical assembly 1826 can be positioned tomeasure changes in the position of reticle (or mask) stage 1816 or othermovable components of the scanner system. Finally, the encoder systemscan be used in a similar fashion with lithography systems involvingsteppers, in addition to, or rather than, scanners.

As is well known in the art, lithography is a critical part ofmanufacturing methods for making semiconducting devices. For example,U.S. Pat. No. 5,483,343 outlines steps for such manufacturing methods.These steps are described below with reference to FIGS. 17A and 17B.FIG. 17A is a flow chart of the sequence of manufacturing asemiconductor device such as a semiconductor chip (e.g., IC or LSI), aliquid crystal panel or a CCD. Step 1951 is a design process fordesigning the circuit of a semiconductor device. Step 1952 is a processfor manufacturing a mask on the basis of the circuit pattern design.Step 1953 is a process for manufacturing a wafer by using a materialsuch as silicon.

Step 1954 is a wafer process that is called a pre-process wherein, byusing the so prepared mask and wafer, circuits are formed on the waferthrough lithography. To form circuits on the wafer that correspond withsufficient spatial resolution those patterns on the mask,interferometric positioning of the lithography tool relative the waferis necessary. The interferometry methods and systems described hereincan be especially useful to improve the effectiveness of the lithographyused in the wafer process.

Step 1955 is an assembling step, which is called a post-process whereinthe wafer processed by step 1954 is formed into semiconductor chips.This step includes assembling (dicing and bonding) and packaging (chipsealing). Step 1956 is an inspection step wherein operability check,durability check and so on of the semiconductor devices produced by step1955 are carried out. With these processes, semiconductor devices arefinished and they are shipped (step 1957).

FIG. 17B is a flow chart showing details of the wafer process. Step 1961is an oxidation process for oxidizing the surface of a wafer. Step 1962is a CVD process for forming an insulating film on the wafer surface.Step 1963 is an electrode forming process for forming electrodes on thewafer by vapor deposition. Step 1964 is an ion implanting process forimplanting ions to the wafer. Step 1965 is a resist process for applyinga resist (photosensitive material) to the wafer. Step 1966 is anexposure process for printing, by exposure (i.e., lithography), thecircuit pattern of the mask on the wafer through the exposure apparatusdescribed above. Once again, as described above, the use of theinterferometry systems and methods described herein improve the accuracyand resolution of such lithography steps.

Step 1967 is a developing process for developing the exposed wafer. Step1968 is an etching process for removing portions other than thedeveloped resist image. Step 1969 is a resist separation process forseparating the resist material remaining on the wafer after beingsubjected to the etching process. By repeating these processes, circuitpatterns are formed and superimposed on the wafer.

The encoder systems described above can also be used in otherapplications in which the relative position of an object needs to bemeasured precisely. For example, in applications in which a write beamsuch as a laser, x-ray, ion, or electron beam, marks a pattern onto asubstrate as either the substrate or beam moves, the encoder systems canbe used to measure the relative movement between the substrate and writebeam.

Other embodiments are in the following claims.

1. A method for determining information about changes along a degree offreedom of an encoder scale, the method comprising: deriving a firstbeam and a second beam from a common source, the first and second beamshaving difference frequencies; directing the first beam and the secondbeam along different paths and combining the first and second beams toform an output beam, where the first beam contacts the encoder scale ata non-Littrow angle and the first beam diffracts from the encoder scaleat least once; detecting an interference signal based on the outputbeam, the interference signal comprising a heterodyne phase related toan optical path difference between the first beam and the second beam;and determining information about a degree of freedom of the encoderscale based on the heterodyne phase.
 2. The method of claim 1, whereinthe first beam is normally incident on the encoder scale.
 3. The methodof claim 1, wherein the first beam is non-normally incident on theencoder scale.
 4. The method of claim 1, wherein the first beam isincident on the encoder scale at an angle so that the diffractedmeasurement beam is normal to the encoder scale.
 5. The method of claim1, wherein the at least one degree of freedom comprises a position ofthe encoder scale along an axis that lies in the plane of the encoderscale.
 6. The method of claim 1, wherein the first and second beams arelinearly polarized beams.
 7. The method of claim 6, wherein the path ofthe first beam before and after diffracting from the encoder scaledefines a plane and the first beam is polarized orthogonal to the plane.8. The method of claim 6, wherein the encoder scale comprises gratinglines extending along a first direction and the measurement beam islinearly polarized in a direction parallel to grating lines.
 9. Themethod of claim 6, wherein directing the first and second beamscomprises rotating a polarization state of the first beam by 90° priorto diffraction from the encoder scale.
 10. The method of claim 1,wherein the first and second beams are orthogonally polarized beams. 11.The method of claim 10, wherein detecting the interference signalcomprises directing the output beam through a polarizing element thattransmits a component of each of the orthogonally polarized first andsecond beams.
 12. The method of claim 1, wherein directing the first andsecond beams along different paths comprises deriving the first andsecond beams from an input beam using a beam splitter.
 13. The method ofclaim 12, wherein the first and second beams are combined using the beamsplitter.
 14. The method of claim 12, wherein the first and second beamsare combined using a second beam splitter.
 15. The method of claim 12,wherein the beam splitter is a polarizing beam splitter.
 16. The methodof claim 1, wherein the second beam does not contact the encoder scale.17. The method of claim 1, wherein the second beam diffracts from theencoder scale at least once.
 18. The method of claim 17, wherein thediffracted second beam is a zero-order diffracted beam.
 19. The methodof claim 17, wherein the diffracted second beam is co-linear with thefirst beam at the encoder scale.
 20. The method of claim 1, wherein thefirst beam diffracts from the encoder scale only once.
 21. The method ofclaim 1, wherein the first beam diffracts from the encoder scale morethan once.
 22. The method of claim 21, wherein a path of the first beamprior to diffracting from the encoder scale is parallel to a path of thefirst beam after diffracting from the encoder scale a second time. 23.The method of claim 21, wherein, after diffracting from the encoderscale a first time, the first beam is directed by a retroreflector todiffract from the encoder scale a second time prior to being combinedwith the second beam.
 24. The method of claim 1, wherein the informationis derived based on more than one heterodyne phase measurements.
 25. Themethod of claim 1, further comprising: combining a third beam and afourth beam to form a second output beam, the third and fourth beamsbeing derived from the common source and the third beam diffracts fromthe encoder scale at least once; and detecting a second interferencesignal based on the second output beam, the second interference signalcomprising a heterodyne phase related to an optical path differencebetween the third beam and the fourth beam.
 26. The method of claim 25,wherein the first and third beams contact the encoder scale at the samelocation.
 27. The method of claim 25, wherein the first and second beamscontact the encoder scale at different locations.
 28. The method ofclaim 25, wherein the information is determined based on the heterodynephases of the first and second output beams.
 29. The method of claim 25,wherein the once-diffracted measurement beam and the third beam are the+1 and −1 diffracted orders of the measurement beam, respectively. 30.The method of claim 1, wherein the at least one degree of freedomcomprises a displacement of the encoder scale along a first axis in theplane of the encoder scale.
 31. The method of claim 30, furthercomprising determining information about a second degree of freedom ofthe encoder scale.
 32. The method of claim 31, wherein the second degreeof freedom is a displacement of the encoder scale along a second axisorthogonal to the first axis.
 33. The method of claim 32, wherein thesecond axis is in the plane of the encoder scale.
 34. The method ofclaim 32, wherein the second axis is orthogonal to the plane of theencoder scale.
 35. The method of claim 1, wherein the at least onedegree of freedom comprises a tilt of the encoder scale about an axis.36. The method of claim 1, further comprising monitoring a referencephase of an input beam produced by the common source and from which thefirst beam is derived.
 37. The method of claim 36, wherein determiningthe information comprises comparing the heterodyne phase to thereference phase.
 38. The method of claim 1, wherein the encoder scalecomprises a grating.
 39. The method of claim 38, wherein the grating hasa pitch in a range from about 1λ, to about 20λ, where λ is a wavelengthof the first beam.
 40. The method of claim 38, wherein the grating has apitch in a range from about 1 μm to about 10 μm.
 41. The method of claim1, wherein the first and second beams are directed along theirrespective paths using an optical assembly and the method furthercomprises translating the encoder scale relative to the optical assemblywhile determining the information.
 42. The method of claim 41, whereinthe optical assembly or encoder scale are attached to a wafer stage andthe method further comprises monitoring the position of a wafer relativeto radiation from a lithography system based on the information.
 43. Themethod of claim 41, wherein the optical assembly or encoder scale areattached to a reticle stage and the method further comprises monitoringthe position of a reticle relative to radiation from a lithographysystem based on the information.
 44. The method of claim 1, wherein thefirst beam has a wavelength in a range from 400 nm to 1,500 nm.
 45. Themethod of claim 1, wherein the first beam has a wavelength of about 633nm or about 980 nm.
 46. A lithography method for use in fabricatingintegrated circuits on a substrate, the method comprising: supportingthe substrate on a moveable stage; imaging spatially patterned radiationonto the substrate; adjusting the position of the stage; using themethod of claim 1 to monitor the position of the stage, wherein theencoder scale or the optical assembly are attached to the stage and theinformation corresponds to the position of the stage along an axis. 47.A lithography method for fabricating integrated circuits on a substratecomprising: positioning a first component of a lithography systemrelative to a second component of a lithography system to expose thesubstrate to spatially patterned radiation; and monitoring the positionof the first component using the method of claim 1, wherein the encoderscale or the optical assembly are attached to the first component andthe information corresponds to the position of the first component. 48.An encoder system, comprising: an optical assembly configured to derivea first beam and a second beam from an input beam, direct the first andsecond beams along different paths and combining the first and secondbeams to form an output beam, where the first and second beams havedifferent frequencies; a diffractive encoder scale extending in a plane,the diffractive encoder scale being positioned in the path of the firstbeam so that the first beam contacts the diffractive encoder scale at anon-Littrow angle and the first beam diffracts from the diffractiveencoder scale at least once; a detector positioned to detect the outputbeam; and an electronic processor configured to receive an interferencesignal from the detector, the interference signal comprising aheterodyne phase related to an optical path difference between the firstand second beams, and determine the information about at least onedegree of freedom of the encoder scale based on the heterodyne phase,wherein the at least one degree of freedom comprises a degree of freedomout of the plane of the encoder scale.
 49. The encoder system of claim48, wherein the optical assembly comprises an optical element thatsplits the input beam into the first and second beams.
 50. The encodersystem of claim 49, wherein the optical element combines the first andsecond beams to form the output beam.
 51. The encoder system of claim49, wherein the optical element is a non-polarizing beam splitter. 52.The encoder system of claim 49, wherein the optical element is apolarizing beam splitter.
 53. The encoder system of claim 48, whereinthe optical assembly comprises one or more optical elements configuredto split the input beam into two parallel sub-input beams prior toderiving the first and second beams.
 54. The encoder system of claim 53,wherein the optical assembly comprises a beam splitter configured tosplit one of the sub-input beams into the first and second beams and thesplit the other sub-input beam into a third and fourth beam, wherein theoptical assembly directs the third and fourth beams along differentpaths and combines the third and fourth beams to form a second outputbeam.
 55. The encoder system of claim 54, wherein the optical assemblydirects the third beam to diffract from the encoder scale at least once.56. The encoder system of claim 55, wherein the optical assemblycomprises two retroreflectors positioned to reflect the once-diffractedfirst and third beams, respectively, to diffract from the encoder scalea second time.
 57. The encoder system of claim 56, wherein the first andthird beams contact the encoder scale at different locations.
 58. Theencoder system of claim 56, wherein the encoder scale diffracts thefirst and third beams into the +1 and −1 diffraction orders,respectively.
 59. The encoder system of claim 54, wherein the path ofthe input beam at the optical assembly is parallel to paths of the firstand second output beams.
 60. The encoder system of claim 59, wherein thepath of the input and first and second output beams are parallel to aplane of the encoder scale.
 61. The encoder system of claim 48, whereinthe optical assembly comprises a half wave plate in the path of thefirst beam.
 62. The encoder system of claim 48, wherein the encoderscale comprises a grating.
 63. The encoder system of claim 62, whereinthe grating is a one-dimensional or a two-dimensional grating.
 64. Asystem, comprising: a moveable stage; and the encoder system of claim48, wherein either the encoder scale or the optical assembly areattached to the stage.
 65. A lithography system for use in fabricatingintegrated circuits on a wafer, the system comprising: a projection lensfor imaging spatially patterned radiation onto the wafer; the encodersystem of claim 48 configured to monitor the position of the waferrelative to the imaged radiation; and a positioning system for adjustingthe position of the stage relative to the imaged radiation, wherein thewafer is supported by the stage.
 66. A lithography system for use infabricating integrated circuits on a wafer, the system comprising: anillumination system including a radiation source, a mask, a positioningsystem, a projection lens, and the encoder system of claim 48, whereinduring operation the source directs radiation through the mask toproduce spatially patterned radiation, the positioning system adjuststhe position of the mask relative to the radiation from the source, theprojection lens images the spatially patterned radiation onto the wafersupported by the stage, and the system monitors the position of the maskrelative to the radiation from the source.